what makes you want iron? would just do all white, or replace it all and do spindles white and banister black.

Wood, wood, wood! Just do wood!

What type of wood banister do you recommendthat would be different from what I already have

I would do the banister with straight square wrought iron and the handrail in dark wood. What you have now looks too chunky for the space.

My grandmother has rod iron and with a house that hasn't changed for 40 years it works but her style house is sooooo elegant, I can't imagine it working there. I think the banister needs a lighter color to go with your wall color. Maybe if you do wood you could make it matching. Or, if the dark color was painted white that could really work too. (do you dislike the white?) I think it goes. It's the dark banister that's off to me.
The carpet can be hard to clean but if you plan to have small children, it's pretty awesome to be a little baby going up and down carpet. I mean, (capital!) I THINK it would be super cool to be a baby on a carpeted stairway. (we had wood, my gran had carpet.) then when you're 3-7 sliding on a sleeping bag!!=HEAVEN!!! LOL
Seriously though, another side note, eye hooks hanging from the under side of the banister---> SUPER COOL for CHristmas Ornaments (if you celebrate). Adds such a pretty look!
good luck!